Hydraulic turbine expanders are used in process plants for the liquefaction of gases, like air, nitrogen, methane, natural gas and other gases, to reduce the enthalpy of the condensed gas and to recover power. U.S. Pat. No. 3,203,191 describes the usage of turbine expanders in gas liquefaction plants and U.S. Pat. No. 5,659,205 specifies a particular design of a hydraulic turbine expander with variable rotational speed.
A process plant for gas liquefaction consists of various elements. Assuming the hydraulic turbine expander to be the imaginary center of the process, then all elements of the liquefaction process, such as compressors, gas expanders, heat exchangers, valves, orifices and pipes can be collectively defined as systems located either upstream or downstream of the turbine expander.
The upstream system is in general designed to cool down and to condense the gas under higher pressure, and the downstream system is designed to handle the liquefied gas.
The downstream system is connected to the terminal vessel, which could be a storage or a phase separator. The terminal vessel is operated with almost constant pressure, independent of the flow rate. The pressure drop in the downstream and upstream system, due to fluid friction, depends significantly on the squared value of the flow rate and on such parameters as density, viscosity, temperature, mixture and inlet conditions for the upstream system, and it is not possible to predict the pressure drop without a certain margin of error.
It is a characteristic of all rotating fluid machines, including hydraulic turbine expanders, that the ratio between output and input power, the efficiency, depends on the value of the potential and kinetic fluid energy, and reaches a maximum value for a certain differential pressure and flow rate. The maximum value is called Best Efficiency Point. For economic reasons it is always advisable to operate fluid machines at the Best Efficiency Point.
Because of the variation and uncertainty range of the pressure drop in the system, it is prior art to install a control valve preferably between the turbine expander and the downstream system, to meet the conditions for a certain differential pressure and flow rate in order to operate the turbine expander at the Best Efficiency Point.
The pressure drop across the control valve is adjustable and expands exactly the necessary pressure difference to meet the value of the terminal pressure, and to allow the turbine expander to operate at the Best Efficiency Point.
The disadvantage of the prior art is that the control valve reduces the pressure through a Joule-Thompson expansion without power recovery.
To optimize the thermodynamic expansion in gas liquefaction processes the overall Joule-Thompson expansion has to be minimized and replaced by expansions which reduce the enthalpy of the gas and liquefied gas.